Induced current measurement systems and methods

ABSTRACT

In an embodiment, the invention includes a measurement system for measuring induced currents within an implantable medical device undergoing magnetic resonance imaging. The measurement system can include a resistor connected in series with a conductive loop and electronic circuitry configured to generate a signal representative of a voltage differential across the resistor. In some embodiments, the measurement system includes a fiber optic cable configured to transmit the signal away from the area subject to magnetic resonance imaging. In some embodiments, the measurement system includes a transmitter to wirelessly transmit the signal away from the area subject to magnetic resonance imaging. In an embodiment, the invention can include an implantable medical device including a measurement system for measuring induced currents. In an embodiment, the invention can include a method of measuring an induced current in an implantable medical device undergoing magnetic resonance imaging. Other embodiments are described herein.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of prior U.S. applicationNo. 11/680,267, filed Feb. 28, 2007, now U.S. Pat. No. 7,873,412,thecontents of which are herein incorporated by reference.

FIELD OF THE INVENTION

The invention relates to the measurement of induced currents, and moreparticularly, to the measurement of induced currents within implantablemedical devices undergoing magnetic resonance imaging (MRI).

BACKGROUND OF THE INVENTION

Many different types of medical devices are implanted within patients toprovide medical therapy. One type of implanted medical device is acardiac rhythm management device, such as a pacemaker or implantabledefibrillator. Cardiac rhythm management devices are used to providemedical therapy to patients who have a disorder related to cardiacrhythm, such as bradycardia.

Magnetic resonance imaging (MRI) is a method of visualizing body tissuesof a patient, primarily to identify pathological conditions or tovisualize physiological structure for purposes of medical diagnosis andtherapy. MRI relies on subjecting the body tissue of interest to a verystrong uniform magnetic field, up to about 30,000 gauss, as well as amoderate strength but variable magnetic field of around 200 gauss. Inthe presence of these uniform and gradient magnetic fields, a radiofrequency (RF) pulse is transmitted from a coil to the body tissue.Hydrogen atoms within the body tissue have a magnetic moment and tend toline up with the direction of the applied magnetic fields. Some of thesehydrogen atoms will align facing one direction and others will alignfacing an opposite direction, such that most of the hydrogen atomsfacing in alternating directions will tend to cancel each other out.However, a small percentage (but a significant absolute number) ofhydrogen atoms will be unbalanced, or not cancelled out. The applied RFpulse tends to cause the unbalanced hydrogen protons to spin, orresonate, in a particular direction and at a particular frequency. Whenthis RF pulse is turned off, the spinning hydrogen protons revert totheir earlier, aligned position, and release their excess energy. The RFcoil of the MRI machine is capable of detecting this emitted energy andtransmitting a corresponding signal to a processor that transforms thesignal to an image of the body tissue. Because different tissues havedifferent characteristic responses to the application of the RF pulse inthe presence of the magnetic fields, these differences can be utilizedto prepare an image showing areas of contrasting tissue types.

MRI techniques have proven to be very effective at diagnosing certainmedical conditions and allowing for patients to receive timely,appropriate medical therapy. However, in many cases patients having animplanted medical device are contraindicated for MRI, and therefore maybe unable to benefit from the full scope of medical treatments availableto them. One problem is that the MRI's RF field can induce a highfrequency current within the implanted device, and this high frequencycurrent can result in tissue heating. In certain circumstances thetissue heating can cause serious injury to the patient. Another andpotentially very serious problem for a patient having certain implantedmedical devices, particularly a cardiac rhythm management device, is thepotential for the MRI machine to create a low frequency (less than 20kHz) induced current (LFIC) in the implanted device. LFIC arises fromthe interaction between the MRI system's time-varying magnetic gradientfields and any conductive loop associated with the implanted device.LFIC in a CRM device can actually cause pacing of the heart byactivating nerve or muscle cells within the heart. In this way, it ispossible for the MRI machine to inadvertently pace the patient's heart.The LFIC can also distort the wave shape of intended pacing pulses,possibly resulting in a diminished effectiveness of the pacing pulse.LFIC can further interfere with the pacemaker system's ability toproperly sense cardiac activity, possibly resulting in inhibited pacingor pacing that is too rapid.

Given the concerns regarding the effects of LFIC in an implanted medicaldevice, it is desired that the LFIC in an implanted device undergoing anMRI be capable of being measured and quantified. Measuring the LFIC inan implantable device may be desirable for the purpose of evaluating theeffects of different device designs on the amount of LFIC generated.Measuring LFIC may also be desirable from the perspective of regulatoryapproval for implanted devices and the need to demonstrate that aparticular device is safe for use in a patient undergoing an MRI. For atleast these reasons, improved techniques for measuring LFIC in animplantable medical device are needed.

SUMMARY OF THE INVENTION

The invention relates to methods and devices for measuring inducedcurrents within implantable medical devices undergoing magneticresonance imaging (MRI). In an embodiment, the invention includes ameasurement system for measuring induced current in an implantablemedical device undergoing magnetic resonance imaging, the measurementsystem including a resistor connected in series with a conductive loopof the implantable medical device, electronic circuitry configured togenerate an electronic signal representative of a voltage differentialacross the resistor and to generate an optical signal corresponding tothe electronic signal, and a fiber optic cable configured to transmitthe optical signal away from an area subject to magnetic resonanceimaging.

In an embodiment, the invention includes a measurement system formeasuring induced current in an implantable medical device undergoingmagnetic resonance imaging, the measurement system including a resistorconnected in series with a conductive loop of the implantable medicaldevice, electronic circuitry configured to generate an electronic signalrepresentative of a voltage differential across the resistor, and atransmitter for wirelessly transmitting the electronic signal away froman area subject to magnetic resonance imaging.

In an embodiment, the invention includes a method of measuring aninduced current in an implantable medical device undergoing magneticresonance imaging, the method including generating an electronic signalrepresentative of the voltage differential across a resistor connectedin series with a conductive loop of the implantable medical device,converting the electronic signal to a corresponding optical signal,transmitting the optical signal through a fiber optic cable to an areanot subject to magnetic resonance imaging, and receiving the opticalsignal in a computing device that is configured to record the signal.

In an embodiment, the invention includes an implantable medical deviceincluding a pulse generator, a lead in communication with the pulsegenerator, the pulse generator and the lead forming part of a conductiveloop, and an induced current sensor including a resistor connected inseries with the conductive loop, and electronic circuitry configured togenerate an electronic signal representative of a voltage differentialacross the resistor.

The invention may be more completely understood by considering thedetailed description of various embodiments of the invention thatfollows in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in connection with thefollowing drawings, in which:

FIG. 1 is a schematic view of a conductive loop formed in an implantedunipolar cardiac pacing device.

FIG. 2 is a schematic view of a conductive loop formed in an implantedbipolar cardiac pacing device.

FIG. 3 is a diagram of an idealized pacing pulse.

FIG. 4 is a diagram of a pacing pulse affected by low frequency inducedcurrent.

FIG. 5 is a schematic diagram of an induced current measurement systemin accordance with an embodiment of the invention, configured for usewith an implantable cardiac rhythm management device.

FIG. 6 is a schematic diagram of an induced current measurement systemin accordance with an embodiment of the invention, configured for usewith a bipolar cardiac rhythm management device.

FIG. 7 is a schematic diagram of electronic components within a sensordevice.

FIG. 8 is a schematic diagram of an alternative embodiment of an inducedcurrent measurement system for use with an implantable cardiac rhythmmanagement device.

While the invention may be modified in many ways, specifics have beenshown by way of example in the drawings and will be described in detail.It should be understood, however, that the intention is not to limit theinvention to the particular embodiments described. On the contrary, theintention is to cover all modifications, equivalents, and alternativesfollowing within the scope and spirit of the invention as defined by theclaims.

DETAILED DESCRIPTION OF THE INVENTION

A variety of implanted medical devices are used to provide medicaltherapies to patients. One example of such an implanted medical deviceis a cardiac rhythm management (CRM) device, used to manage cardiacconditions such as bradycardia and tachycardia. A specific example of aCRM device is a pacemaker, which can include a pulse generator forgenerating a pacing pulse and one or more leads for delivering thepacing pulse to the cardiac tissue. Pacemakers can be configured tosense the electrical activity of the patient's heart, as transmittedthrough the leads. In some pacing modes, if the pacemaker does notdetect electrical activity above a certain trigger threshold within acertain time interval, the pacemaker will deliver a pacing pulse throughthe one or more leads to the cardiac tissue. This pacing pulse causesthe heart to beat.

Magnetic resonance imaging relies on the creation of time varyingmagnetic field gradients within a patient's body. The body of a patientundergoing an MRI exam is not subject to a uniform magnetic field, butrather is subject to a magnetic field that is different at each locationof the patient's body and that varies continuously with time. Faraday'slaw states that any change in a magnetic field around a conductive loopwill cause a voltage to be induced in the conductive loop, andconsequently, cause a current to flow in the conductive loop. As perFaraday's law, the time varying magnetic field gradients in the body ofa patient undergoing an MRI procedure generate a voltage, andconsequently a current, in any conductive loop present within the timevarying magnetic field. In the case of a patient having an implanted CRMdevice and undergoing an MRI procedure, the time varying magnetic fieldgradient of the MRI machine creates the required changing magnetic fieldand the implanted pacemaker or other cardiac rhythm management deviceforms the conductive loop. The induced currents can include lowfrequency induced currents (LFICs), such as at a frequency of less than20 kHz, that can interfere with the functioning of an implanted medicaldevice. For example, it is possible that LFIC could cause pacing of theheart by activating nerve or muscle cells within the heart. In this way,it may be possible for the MRI machine to inadvertently pace thepatient's heart. The LFIC can also distort the waveshape of intendedpacing pulses, possibly resulting in a diminished effectiveness of thepacing pulse. LFIC can further interfere with the pacemaker system'sability to properly sense cardiac activity, possibly resulting ininhibited pacing or rapid pacing.

In a unipolar pacemaker system such as that depicted in FIG. 1, a loop20 is formed from the pacemaker internal circuitry 22, through the lead24 to the electrode 26 in contact with cardiac tissue, and then throughbody tissue back to the pacemaker housing 28. The area enclosed by thisloop is significant and therefore a substantial amount of LFIC can begenerated within this loop by the time varying magnetic field gradientsof an MRI system.

Conductive loops can also be created in the context of bipolar pacingsystems. FIG. 2 shows a simplified schematic diagram of some aspects ofa typical bipolar pacemaker system. Bipolar pacemaker 54 includes a tipand ring electrode 32, where the tip electrode 34 and ring electrode 36are each implanted in cardiac tissue, but are separated by a relativelysmall distance from each other. Pacemaker 54 can include variouscircuitries, such as pulse generation circuitry, sensing circuitry,charging circuitry, control circuitry, and the like. Sensing circuitry,charging circuitry, and control circuitry (not shown in FIG. 2) can beconstructed according to principles known to those of skill in the art.In FIG. 2, pulse generator 38 includes pacing switch S_(p), pacingcapacitor C_(p), recharging switch S_(r), and recharging capacitorC_(r). A housing 44 is provided that contains pulse generator 38. Thehousing 44 can be constructed of a conductive material.

As shown in FIG. 2, pulse generator 38 also includes switch S_(m) forswitching between bipolar mode and unipolar mode. To select a unipolarmode of operation, switch S_(m) is configured to connect the pacemakerhousing 44 to the pulse generator 38 circuitry. In the unipolar mode ofoperation, the tip electrode 34 generally serves as the cathode and thehousing 44 itself serves as the anode. In FIG. 2, this occurs whereswitch S_(m) connects to terminal 2 of switch S_(m). To select a bipolaroperation mode, switch S_(m) is configured to connect conductor 42 tothe pulse generator 38 circuitry. In the bipolar mode of operation, thetip electrode 34 generally serves as the cathode and the ring electrode36 generally serves as the anode. In the embodiment of FIG. 2, thisoccurs when switch S_(m) connects to terminal 1 of switch S_(m).

In bipolar capable pacemakers, there is generally more than oneconductive loop in which current can be induced. In bipolar mode, afirst loop 46 is formed when either switch S_(p) or switch S_(r) isclosed, the first loop 46 being formed either through switch S_(r) orcapacitor C_(p) and switch S_(p), through capacitor C_(r), through firstconductor 40 and tip electrode 34, through cardiac tissue into ringelectrode 36, and through second conductor 42 to switch S_(m). However,first and second conductors 40, 42 are generally very close together,such as disposed together within one lead. Therefore, conductive loopsthat include both first conductor 40 and second conductor 42 generallyenclose a very small area and therefore induced current in these loopsis usually insignificant.

However, conductive loops enclosing a relatively large area can also beformed by some bipolar pacemakers. Many bipolar pacemakers include anintegrated circuit protection diode D₁. Diode D₁ allows current to flowfrom the pacemaker housing 44 into the pulse generator circuitry to thereference potential (ground) of capacitor C_(p). This is useful toprevent the pacemaker ground from deviating from the pacemaker housingpotential. However, this diode D₁ can facilitate the formation ofconductive loops within the pacemaker. For example, when switch S_(p) isclosed, loop 48 is formed passing through capacitor C_(p), switch S_(p),capacitor C_(r), conductor 40, tip electrode 34, tissue path 50, back tohousing 44 and through diode D₁. When switch Sr is closed, loop 49 isformed passing through switch Sr, capacitor Cr, conductor 40, tipelectrode 34, tissue path 50, back to housing 44 and through diode D₁.Loops 48 and 49 can be formed regardless of the position of switchS_(m).

Furthermore, when switch S_(m) is in bipolar mode, another conductiveloop 52 can be formed regardless of the positions of switches S_(r) andS_(p). Conductive loop 52 can be formed passing through second conductor42, electrode 36, tissue path 50 to housing 44, through diode D₁, andback to second conductor 42 through switch S_(m). Loops 48, 49, and 52each enclose an area sufficiently large to make the generation of LFICduring MRI a concern.

LFIC can have harmful effects on the patient. If the induced current islarge enough, the current can cause activation of the heart muscle. Theinduced current can also cause distortion of a pacing pulse sent fromthe pacemaker through the leads to the heart. For example, FIG. 3 showsan example of an idealized pacing pulse. At a first time T₁, a pacingswitch is closed causing a current pulse to be delivered through theleads for a period of time, until at time T₂ the pacing switch is openedand the current pulse diminishes. Also at time T₂, a charging switch isclosed to allow charging of a capacitor until time Tt₃ when the chargingswitch is opened. FIG. 4 shows an example of how an idealized pacingpulse can be affected by the presence of LFIC. The current that isinduced into the loop will add to or subtract from the voltage of thepacing pulse, resulting in a distorted pulse, such as that seen in FIG.4. In some cases, the induced distortion may cause the electrical pulseto be insufficient to capture the patient's heart. Alternatively, theinduced distortion may result in unreliable sensing of electricalactivity in the heart. In some cases, the LFIC may be large enough inmagnitude to capture the patient's heart at times other than during thepacing pulse. For example, by forward biasing diode D₁, capture outsideof the pace and active recharge window can be facilitated. In any case,the LFIC can interfere with the proper operation of the pacing device,possibly causing injury to the patient.

Embodiments of LFIC measurement systems according to the presentinvention can include features for accurately measuring LFIC. Forexample, in at least some embodiments, the induced current is measuredat two locations very close together to minimize the effect of thegradient magnetic fields on the measurement.

An embodiment of a measurement system constructed according to theprinciples of the present invention is depicted in FIG. 5. Medicaldevice 140 is implantable in a patient. In the embodiment of FIG. 5,medical device 140 is a unipolar CRM device. However, other devices areusable, such as a bipolar CRM device. The CRM device 140 of FIG. 5includes a housing 142 containing electronic circuitry, at least oneelectrically conductive lead 144, and at least one electrode 146configured to be attached to the patient's cardiac tissue. A conductiveloop is formed from housing 142 and associated circuitry, through lead144 and electrode 146, and through tissue path 148 back to housing 142.An induced current measurement system 150 is provided thatadvantageously incorporates two measurement locations very closetogether, such as on opposite sides of a single small resistor. In oneembodiment, the measurement locations are less than 25 mm apart. Inanother embodiment, the measurement locations are less than 10 mm apart.In yet another embodiment, the measurement locations are less than 5 mmapart. By placing the measurement locations close together, the effectof the magnetic field gradient associated with MRI is greatlydiminished.

Induced current measurement system 150 includes a current sensor device152 positioned in series with the conductive loop associated withimplantable medical device 140. In the embodiment of FIG. 5, currentsensor device 152 is generally positioned in series with lead 144.Alternatively, current sensor device 152 could be positioned in seriesbetween lead 144 and housing 142 and associated circuitry. As anotheralternative, current sensor device 152 could be positioned in serieswith lead 144 and disposed within housing 142. By being positioned inseries with an element of the conductive loop, any current, includingboth induced current and, in the case of a CRM, current pulses deliveredby the CRM to pace the heart, will pass through sensor device 152.Current sensor device 152 can include a resistor 154, a power supply156, and sensor electronic circuitry 158. Resistor 154 can be a resistorhaving very small resistance, on the order of 10 ohms. In otherembodiments, however, resistor 154 has a resistance between 1 ohm and 50ohms.

Another embodiment of a measurement system constructed according to theprinciples of the present invention is depicted in FIG. 6. Inducedcurrent measurement system 100 is configured for use with implantablecardiac rhythm management device 102. Device 102 is configured forbipolar pacing, and is constructed similarly to the device depicted inFIG. 2 with the exception that the induced current measurement system100 has two independent measurement channels, one channel for eachconductor 40, 42. As discussed above, two separate loops may be formedwithin a bipolar pacing device, where one loop includes first conductor40 and where the other loop includes the second conductor 42. Therefore,it can be desirable to measure the current within each conductor 40, 42.Induced current measurement system 100 includes current sensor device104 positioned in series with each of conductors 40, 42. Current sensordevice 104 includes first resistor 106 positioned in series with firstconductor 40 and second resistor 108 positioned in series with secondconductor 42. Current sensor device 104 also includes a power supply110, first channel sensor electronic circuitry 112, and second channelsensor electronic circuitry 114. Resistors 106, 108 can be resistorshaving very small resistance, on the order of 10 ohms. In otherembodiments, however, resistors 106, 108 have a resistance between 1 ohmand 50 ohms.

Each of sensor electronic circuitry 158, first channel sensor electroniccircuitry 112, and second channel sensor electronic circuitry 114 can beconstructed similarly. For ease of description, reference will only bemade to sensor electronic circuitry 158. However, it is to beappreciated that the description of sensor electronic circuitry 158 alsoapplies to first channel sensor electronic circuitry 112 and secondchannel sensor electronic circuitry 114. Sensor electronic circuitry 158is configured to generate a signal representative of the induced currentwithin the conductive loop, process that signal, and provide aninterface to transmit the signal to an area outside of the MRIenvironment. One embodiment of sensor electronic circuitry 158 is shownin FIG. 7. In this embodiment, the sensor electronic circuitry 158includes sensing circuitry 70, amplification circuitry 72, filtrationcircuitry 74, digitization circuitry 76, and interface circuitry 78.Various aspects of circuitries 70, 72, 74, 76, 78 may occur inphysically separate components or circuits, may occur in a single areaor component, or may occur in a microprocessor or microcontroller.Various aspects of circuitries 70, 72, 74, 76, 78 may occursequentially, simultaneously, or in another order.

Sensing circuitry 70 is configured to generate an analog signalcorresponding to the voltage differential between a first side 154 a ofresistor 154 and a second side 154 b of resistor 154, where this signaltherefore also corresponds to the current passing through resistor 154according to the relationship defined in Ohm's Law. Digitizationcircuitry 76 is generally configured to convert the analog signal to acorresponding digital signal, and generally comprises an analog todigital converter. In an embodiment, digitization circuitry 76 has asampling rate of 20 kHz or higher. In some embodiments, digitizationcircuitry 76 has a sampling rate of 40 kHz or higher. In an embodiment,digitization circuitry 76 has a sampling resolution of at least 8 bits.Although digitization circuitry 76 can have a different relationshipwith other circuitries 70, 72, 74, 78 and occur in different orders, insome embodiments digitization occurs as close to the signal generationas possible to limit signal interference and degradation from the MRImagnetic fields. Amplification circuitry 72 is generally configured toamplify the signal generated in the sensing circuitry 70. Filtrationcircuitry 74 is configured to filter the signal as necessary. In oneembodiment, filtration circuitry 74 is configured to achieve adifferential signal bandwidth of 15 kHz, high common mode rejection ofaround 80 dB, and sufficient common mode electromagnetic interferencefiltering to avoid corruption from rectified high frequency fields suchas the MRI's RF field. Interface circuitry 78 is configured to interfacewith the environment outside of the MRI machine, generally includingtransmitting the current signal and receiving start and stopinstructions. In an embodiment, the interface circuitry 78 includes anoptical transmission device, such as a light emitting diode, forconverting an electronic signal to an optical signal. Interfacecircuitry 78 may also perform other functions related to datatransmission, such as data compression, coding, or modulation. In someembodiments, particularly embodiments having more than one measurementchannel, interface circuitry 78 includes a universal asynchronousreceiver/transmitter (UART) for managing the transmission of the digitalsignal. In one embodiment, a bi-phase UART is provided for digitalcoding and modulation for transmission over a fiber optic channel. AUART typically is provided to convert parallel data streams, such asdata streams from two measurement channels, to a serial data stream fortransmission. Each of these circuitries (70, 72 74, 76, 78) can beconstructed according to general principles known to those of skill inthe art. Further, power supply 156 is incorporated as necessary for thefunctioning of these circuitries.

In some embodiments, a fiber optic cable 60 is provided that is insignal communication with interface circuitry 78. In some embodiments,two or more fiber optic cables 60 are provided. Referring back to FIG.5, each fiber optic cable 60 can be configured to pass out of the MRImachine and through a magnetic shielding wall 62 that defines the limitsof the MRI testing environment. On the opposite side of shielding wall62, fiber optic cable 60 is in signal communication with a computingdevice 64.

In other embodiments interface circuitry 78 is configured to wirelesslytransmit the data signal out of the MRI environment. For example,interface circuitry 78 may include a wireless transmitter, such as aradio frequency transmitter, for transmitting the data signal out of theMRI environment.

Referring back to FIG. 5, in one embodiment, computing device 64 is adigital computer configured to receive optical signals transmittedthrough fiber optic cable 60, convert the optical signals to digitalsignals, and process the digital signals. In some embodiments, computingdevice 64 is configured to process the signal to calculate an inducedcurrent level within the implantable device 140 that corresponds to themeasured voltage differential. For example, the computing device 64 maycorrelate the received optical signal to a voltage differential acrossresistor 154, and divide this voltage differential by the resistance ofthe resistor 154 to determine the amount of current flowing within theconductive loop of device 140.

In an embodiment, computing device 64 includes an interface unit that isconfigured to receive an optical data stream, perform other functionssuch as error correction and channel demultiplexing, and to communicatethe data stream to a digital computer for further processing. In yetanother embodiment, computing device 64 includes an interface unit thatis configured to receive wireless signals from transmission circuitry78. In an embodiment, an interface unit is separate from a digitalcomputer, and includes a microcontroller, a fiber optic transmit andreceive module, a UART, and a USB interface for establishing acommunication channel with the digital computer. In another embodiment,computing device 64 and an interface unit are not separate components.In yet another embodiment, computing device 64 is configured to receiveinput and generate commands, such as start or stop, that may betransmitted through fiber optic cable 60 to transmission circuitry 78.In one embodiment, computing device 64 consists of a single device. Inother embodiments, computing device 64 consists of more than one device,where the various devices function together.

In some embodiments, the computing device 64 is configured to create arepresentation of the induced current within the implantable medicaldevice, such as a waveform or a depiction of the amount of inducedcurrent. In this manner, the characteristics of the LFIC within theimplanted device can be determined. This information can be used toevaluate various device designs, such as for determining the effect aproposed device design has on LFIC. This information can also be used todetermine whether there is a risk of adverse effects when a patienthaving an implanted device is subjected to an MRI examination, and alsofor the purposes of demonstrating the safety of the implanted device andattaining regulatory approval for the use of the implanted device in anMRI machine.

The current measurement system 150 and implantable medical device 140may be positioned either in vitro or in vivo for testing purposes, asdictated by the nature of the study being performed. If system 150 anddevice 140 are in vitro, it is necessary to simulate a conductive tissuepath 148. For example, tissue path 148 could be simulated by a resistorbetween electrode 146 and housing 142 that is comparable to theresistance of body tissue between these two points. The implantablemedical device 140 and the current sensor device 152 can then be placedwithin the MRI machine, with fiber optic cable 60 passing out of the MRImachine and out of the MRI room. The MRI machine is then activated andutilized in a customary manner.

Embodiments of the invention can also include other features. By way ofexample, in some embodiments, the measurement circuitry does not includeany loops that could themselves be subject to current induction. In somecases, the measurement circuitry includes no ferromagnetic materials inorder to minimize the influence of the magnetic fields in the MRI. Insome embodiments, the measurement circuitry includes analog circuitrythat is physically very small to minimize signal distortion. In someembodiments, the system includes one or more non-conductivecommunication channels for transmitting sensed current data out of theMRI environment without being subject to magnetic interference.

Yet another embodiment is depicted in FIG. 8. The embodiment of FIG. 8includes elements that are similar to the embodiment depicted in FIG. 2,such as a pulse generator 38 inside a housing 44, and has additionalcomponents that function similarly to the embodiment depicted in FIG. 2.However, the embodiment of FIG. 8 further includes an induced currentmeasurement system 200 within the housing 44. In one embodiment, theinduced current measurement system 200 includes a first resistor 202positioned in series with first conductor 40 and a second resistor 204positioned in series with second conductor 42. Other embodiments haveonly one of first resistor 202 and second resistor 204. By positioningeither of first resistor 202 and second resistor 204, or both, in serieswith an element of the conductive loop such as first or secondconductors 40, 42, any current, including both induced current and, inthe case of a CRM, current pulses delivered by the CRM to pace theheart, will pass through the resistors 202, 204. Current sensor system200 includes voltage sensing leads 208, 210, 212, 214 that transmit thevoltage at each side of resistor 202, 204, respectively, to sensorelectronic circuitry 206. Resistors 202, 204 can be resistors havingrelatively small resistance, on the order of 10 ohms. In otherembodiments, however, resistors 202, 204 have a resistance between 1 ohmand 50 ohms.

Sensor electronic circuitry 206 senses the voltage at each side ofresistors 202, 204, as present. Based on the measured voltagedifferential, and also the known resistance of resistors 202, 204,sensor electronic circuitry can determine the amount of current in firstand second conductors 40, 42, respectively. In the embodiment of FIG. 8,sensor electronic circuitry 206 is further configured to store or recordthe measured voltage differentials or currents. The sensor electroniccircuitry 206 may also include telemetry capabilities to allow forwireless communication between the sensor electronic circuitry 206 andan external device. The stored data can be transmitted to the externaldevice as desired or convenient, such as after a patient completes anMRI examination or at a patient's regularly scheduled check-up. In otherembodiments, the sensor electronic circuitry 206 can wirelessly transmitthe voltage or current data to an external device in real time while apatient undergoes an MRI procedure.

It will be appreciated that an induced current measurement system canalso be located within a header of an implantable device.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the present specification. Theclaims are intended to cover such modifications and devices.

The above specification provides a complete description of the structureand use of the invention. Since many of the embodiments of the inventioncan be made without parting from the spirit and scope of the invention,the invention resides in the claims.

1. A measurement system for measuring induced current in an implantablemedical device undergoing magnetic resonance imaging, the measurementsystem comprising an implantable medical device comprising: a resistor;a conductive loop comprising a pulse generator and a lead, theconductive loop in series with the resistor; electronic circuitryconfigured to generate an electronic signal representative of a voltagedifferential across the resistor; and telemetry circuitry configured toallow the implantable medical device to communicate with an externaldevice.
 2. The measurement system of claim 1, further comprising anexternal device configured to receive a wireless signal from theimplantable medical device.
 3. The measurement system of claim 1,wherein the resistor has a resistance of between about 1 ohms and about50 ohms.
 4. The measurement system of claim 3, the resistor connected inseries between the pulse generator and the lead.
 5. The measurementsystem of claim 1, the electronic signal representing a voltagedifferential between two locations less than 5 mm apart.
 6. Themeasurement system of claim 1, the electronic circuitry comprising ananalog to digital converter.
 7. The measurement system of claim 2,wherein the computing device is further configured to determine aninduced current level from the voltage differential signal.
 8. Themeasurement system of claim 2, the induced current having a frequency ofless than about 20 kHz.
 9. A method of measuring an induced current inan implantable medical device undergoing magnetic resonance imaging, themethod comprising: (i) using an implantable medical device to generatean electronic signal representative of the voltage differential across aresistor connected in series with a conductive loop in the implantablemedical device, wherein the conductive loop comprises a pulse generatorand a lead; and (ii) wirelessly communicating the signal from theimplantable medical device to an external device using wirelesstelemetry.
 10. The method of claim 9, the resistor connected in seriesbetween a pulse generator and the lead.
 11. The method of claim 9, theelectronic voltage differential signal representing the voltagedifferential between two locations less than 5 mm apart.
 12. The methodof claim 9, further comprising the step of converting the electronicsignal from analog to digital.
 13. The method of claim 9, the computingdevice further configured to determine an induced current level from thevoltage differential signal.
 14. The method of claim 9, wherein thewireless communication between the implantable medical device and theexternal device occurs in real time while a patient undergoes an MRIprocedure.
 15. A measurement system for measuring induced current in animplantable medical device undergoing magnetic resonance imaging, themeasurement system comprising: (i) a lead; (ii) a resistor configured tobe connected in series with a conductive loop of the implantable medicaldevice defined by the lead and a pulse generator, the resistor disposedoutside of the pulse generator; (iii) electronic circuitry configured togenerate an electronic signal representative of a voltage differentialacross the resistor and to generate an optical signal corresponding tothe electronic signal; and (iv) a fiber optic cable configured totransmit the optical signal away from an area subject to magneticresonance imaging.